Heavy Lift, Deck Load, and Counter-Ballast
Your 9,000 t displacement general cargo ship has a KM of 8.20 m and a solid KG of 7.10 m. A 120 t packaged unit is to be lifted from the quay by the ship's crane and stowed on deck. The crane hook is 4.0 m to starboard of the centreline and the unit's final Kg is 10.50 m. At the same time, a double-bottom ballast tank to port is to be filled to counteract list; the transfer will put 100 t at 3.0 m to port, and the partly filled tank creates a free surface moment of 600 t·m. The ship's present free surface correction from other tanks is 0.05 m. The deck load limit is 2.5 t/m² and the unit's footprint is 4.0 m × 2.5 m. The stability booklet requires a minimum GM of 0.30 m and a maximum list of 5° alongside.
1. Calculate the initial GM, the new displacement and KG, the GM after loading and free surface, and the list during the lift. Compare each result with the limits.
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Initial: GM solid = KM − KG = 8.20 − 7.10 = 1.10 m. With the existing FSC 0.05 m, initial GM fluid = 1.05 m.
After loading: displacement = 9,000 + 120 = 9,120 t. New KG = (9,000 × 7.10 + 120 × 10.50) / 9,120 = (63,900 + 1,260) / 9,120 = 7.145 m. New GM solid = 8.20 − 7.145 = 1.055 m. Existing FSM = 0.05 × 9,000 = 450 t·m; add 600 t·m gives 1,050 t·m. FSC = 1,050 / 9,120 = 0.115 m. GM fluid = 1.055 − 0.115 = 0.94 m, which is above the 0.30 m minimum.
List during the lift: the unit hangs at 4.0 m to starboard, so its transverse moment is 120 × 4.0 = 480 t·m to starboard. The ballast gives 100 × 3.0 = 300 t·m to port. Net = 180 t·m to starboard. tan θ = 180 / (9,120 × 0.94) = 0.0210; θ ≈ 1.2° to starboard. That is within the 5° limit, but the list will be larger if the ballast is not yet transferred: with no ballast, tan θ = 480 / (9,120 × 0.94) = 0.056; θ ≈ 3.2°, still within 5° but with less margin. The critical case is during the lift, not after the unit is set down.
2. Check the deck load and decide whether the stowage is acceptable. Propose a solution.
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Deck load = 120 t / (4.0 × 2.5) = 12 t/m², which is 4.8 times the 2.5 t/m² limit. The stowage as proposed is not acceptable on the bare deck. The load must be spread over at least 120 / 2.5 = 48 m², for example an 8 m × 6 m grillage or equivalent beams, and the deck framing and point loads must be checked. The crane's load chart, radius, sea state and lashings also need checking. I would not lift until a load-spreading arrangement is approved by the company/naval architect and the deck is confirmed capable of taking the concentrated load. A smaller footprint with a bigger grillage can still fail if the deck longitudinals are not strong enough, so the check is not just area.
3. Set out the ballast and lift sequence to keep list within 2°, and list the stability checks before the lift.
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Pre-ballast the port double-bottom in steps before the lift, monitoring heel and draft; do not start a large ballast transfer with the load already hanging. Keep the unit low and control the lift with tag lines; stop if the list approaches 2°. Fill the port tank to give a small counter-moment, but remember that the 600 t·m free surface already reduces GM. Before the lift: confirm GM fluid ≥0.30 m, list ≤2°, trim suitable for the crane, deck load spread approved, crane load chart and radius checked, moorings slackened or adjusted, and stability computer updated. During the lift: monitor heel, draft, crane load, and weather; keep the load close to the deck and be ready to set it down. After stowage: recheck list, GM and lashings, and record the final condition.